Okay, now we have a legitimate issue to talk about.
However, I think this is to a great portion caused by the playtest having only stats for critters and dumb brutes who simply soak damage instead of avoiding damage. Which I don't think are the types of enemies this ability has been made for. It gets interesting once you get to enemies that are hard to hit.
Admitedly, putting Glancing Blow in this playtest package where there are no such encounters wasn't such a smart idea.
Yes, any enemy or condition that would cause you to miss even though you hit the required AC could be partially countered
I'm not convinced that is how we've ended up here.
What I'm seeing is the Slayer's mechanic from playtest 1, watered down due to feedback from testers that didn't like the auto-damage mechanic. Unfortunately it's been over-corrected (or badly written possibly IMO) to the point of being ineffective.